Like organisms evolved in gentle tide pools, who migrate to freezing
oceans or steaming jungles by developing metabolisms, mechanisms, and
behaviors workable in those harsher and vaster environments, our
descendants, able to change their representations at will, may develop
means to venture far from the comfortable realms we consider reality
into arbitrarily strange worlds. Their techniques will be as
meaningless to us as bicycles are to fish, but perhaps we can stretch
our common-sense-hobbled imaginations enough to peer a short distance
into this odd territory.

Simulation, Consciousness, ExistenceHans Moravec, 1998

Simulation

During the last few centuries, physical science has convincingly
answered so many questions about the nature of things, and so hugely
increased our abilities, that many see it as the only legitimate
claimant to the title of true knowledge. Other belief systems may
have social utility for the groups that practice them, but ultimately
they are just made-up stories. I myself am partial to such
``physical fundamentalism.''

Physical fundamentalists, however, must agree with René
Descartes that the world we perceive through our senses could be an
elaborate hoax. In the seventeenth century Descartes considered the
possibility of an evil demon who created the illusion of an external
reality by controlling all that we see and hear (and feel and smell
and taste). In the twenty-first century, physical science itself,
through the technology of virtual reality, will provide the means to
create such illusions. Enthusiastic video gamers and other cybernauts
are already strapping themselves into virtual reality goggles and body
suits for brief stints in made-up worlds whose fundamental mechanisms
are completely different from the quantum fields that (best evidence
suggests) constitute our physical world.

Today's virtual adventurers do not fully escape the physical world:
if they bump into real objects, they feel real pain. That link may
weaken when direct connections to the nervous system become possible,
leading perhaps to the old science-fiction idea of a living brain in a
vat. The brain would be physically sustained by life-support
machinery, and mentally by connections of all the peripheral nerves to
an elaborate simulation of not only a surrounding world but also a
body for the brain to inhabit. Brain vats might be medical stopgaps
for accident victims with bodies damaged beyond repair, pending the
acquisition, growth, or manufacture of a new body.

The virtual life of a brain in a vat can still be subtly perturbed by
external physical, chemical, or electrical effects impinging on the
vat. Even these weak ties to the physical world would fade if the
brain, as well as the body, was absorbed into the simulation. If
damaged or endangered parts of the brain, like the body, could be
replaced with functionally equivalent simulations, some individuals
could survive total physical destruction to find themselves alive as
pure computer simulations in virtual worlds.

A simulated world hosting a simulated person can be a closed
self-contained entity. It might exist as a program on a computer
processing data quietly in some dark corner, giving no external hint
of the joys and pains, successes and frustrations of the person
inside. Inside the simulation events unfold according to the strict
logic of the program, which defines the ``laws of physics'' of the
simulation. The inhabitant might, by patient experimentation and
inference, deduce some representation of the simulation laws, but not
the nature or even existence of the simulating computer. The
simulation's internal relationships would be the same if the program
were running correctly on any of an endless variety of possible
computers, slowly, quickly, intermittently, or even backwards and
forwards in time, with the data stored as charges on chips, marks on a
tape, or pulses in a delay line, with the simulation's numbers
represented in binary, decimal, or Roman numerals, compactly or spread
widely across the machine. There is no limit, in principle, on how
indirect the relationship between simulation and simulated can be.

Today's simulations, say of aircraft flight or the weather, are
run to provide answers and images. They do so through additional
programs that translate the simulation's internal
representations into forms convenient for external human observers.
The need to interpret limits how radical a simulation's hardware
and software representations can be. Making them too different from
the form of the answers may render the translation impractically slow
and expensive. This practical limit may be irrelevant for
simulations, such as the medical rescue imagined above, that contain
their own observers. Conscious inhabitants of simulations experience
their virtual lives whether or not outsiders manage to view them.
They can be implemented in any way at all.

What does it mean for a process to implement, or encode, a
simulation? Something is palpably an encoding if there is a way of
decoding or translating it into a recognizable form. Programs that
produce pictures of evolving cloud cover from weather simulations, or
cockpit views from flight simulations, are examples of such decodings.
As the relationship between the elements inside the simulator and the
external representation becomes more complicated, the decoding process
may become impractically expensive. Yet there is no obvious cutoff
point. A translation that is impractical today may be possible
tomorrow given more powerful computers, some yet undiscovered
mathematical approach, or perhaps an alien translator. Like people
who dismiss speech and signs in unfamiliar foreign languages as
meaningless gibberish, we are likely to be rudely surprised if we
dismiss possible interpretations simply because we can't achieve them
at the moment. Why not accept all mathematically possible decodings,
regardless of present or future practicality? This seems a safe,
open-minded approach, but it leads into strange territory.

An interpretation of a simulation is just a mathematical mapping
between states of the simulation process and views of the simulation
meaningful to a particular observer. A small, fast program to do
this makes the interpretation practical. Mathematically, however,
the job can also be done by a huge theoretical lookup table that
contains an observer's view for every possible state of the
simulation.

The observation is disturbing because there is always a table that
takes any particular situation---for instance, the idle passage of
time---into any sequence of views. Not just hard-working computers,
but anything at all can theoretically be viewed as a simulation of any
possible world! We are unlikely to experience more than an
infinitesimal fraction of the infinity of possible worlds, yet, as our
ability to process data increases, more and more of them will become
potentially viewable. Our ever-more superintelligent progeny will be
able to make increasingly huge interpretive leaps, far beyond anything
now imaginable. But whether or not they are ever seen from outside,
all the possible worlds are as physically real to any conscious
inhabitants they may contain as our world is to us.

This line of thought, growing out of the premises and techniques of
physical science, has the unexpected consequence of demoting physical
existence to a derivative role. A possible world is as real, and only
as real, as conscious observers, especially inside the world, think it
is!

Consciousness

But what is consciousness? The prescientific suggestion that humans
derive their experience of existence from spiritual mechanisms outside
the physical world has had notable social consequences, but no success
as a scientific hypothesis. Physical science has only recently begun
to address the question on its own terms, from vantage points
including evolutionary biology, anthropology, psychology,
neurobiology, and computer science.

Human consciousness may be a by-product of a brain evolved for social
living. Memory, prediction and communication mechanisms, similar but
distinct from those for keeping track of physical objects, evolved to
classify and communicate the moods and relations of tribe members.
Aggressive and submissive behaviors, for instance, just like bad and
good smells, became classified into categories linked to behavioral
responses and also communicable symbols. As language evolved, it
became possible to tell stories about both physical and psychological
events. At some point, perhaps very early in its evolution, the
storytelling mechanism was turned back on the teller, and the story
began to include commentary about the teller's state of mind along
with the external happenings.

Our consciousness may be primarily the continuous story we tell
ourselves, from moment to moment, about what we did and why we did it.
It is a thin, often inaccurate veneer rationalizing a mountain of
unconscious processing. Not only is our consciousness-story a weak
reflection of physical and brain reality, but its very existence is a
purely subjective attribution. Viewed from the physical outside, the
story is just a pattern of electrochemical events, probably in mainly
our left cortex. A complex psychological interpretation must be
invoked to translate that pattern into a meaningful tale. From the
psychological inside, the story is compelling because the
psychological interpretation is an essential element of the story, its
relationships enforced unconsciously by the interconnections of the
storytelling neural machinery.

On the one hand, our consciousness may be an evolutionary fluke,
telling an unreliable story in a far-fetched interpretation of a
pattern of tiny salty squirts. On the other, our consciousness is
the only reason for thinking we exist (or for thinking we think).
Without it there are no beliefs, no sensations, no experience of
being, no universe.

Existence

What is reality, anyway? The idea of a simulated existence is the
first link in our disturbing chain of thought. Just as a literary
description of a place can exist in different languages, phrasings,
printing styles, and physical media, a simulation of a world can be
implemented in radically different data structures, processing steps,
and hardware. If one interrupts a simulation running on one machine
and translates its data and program to carry on in a totally
dissimilar computer, the simulation's intrinsics, including the
mental activity of any inhabitants, continue blithely to follow the
simulated physical laws. Only observers outside the simulation
notice if the new machine runs at a different speed, does its steps
in a scrambled order, or requires elaborate translation to make sense
of its action.

A simulation, say of the weather, can be viewed as a set of numbers
being transformed incrementally into other numbers. Most computer
simulations have separate viewing programs that interpret the internal
numbers into externally meaningful form, say pictures of evolving
cloud patterns. The simulation, however, proceeds with or without
such external interpretation. If a simulation's data representation
is transformed, the computer running it steps through an entirely
different number sequence, although a correspondingly modified viewing
program will produce the same pictures. There is no objective limit
to how radical the representation can be, and any simulation can be
found in any sequence, given the right interpretation. A simple clock
simulates the evolving state of a complex world when interpreted via a
world-describing playbook or movie frames keyed to clock ticks. Even
the clock is superfluous, since an external observer can read the book
or watch the movie at any pace. If the interpretation of a simulation
is a dispensable external, while its core implementation can be
transformed away to nothing, in what sense can a simulated world be
said to exist at all?

Mathematical realism, a philosophical position advocated by Plato,
illuminates this problem's vexing intangibles. Mathematical objects
like numbers and geometric shapes manifest themselves just as richly
and consistently to abstract thought as physical objects impress the
senses. To Plato, mathematical concepts were as real as physical
objects, just invisible to the external senses as sound is
imperceptible to the eyes.

Computer simulation brings mathematical realism neatly full circle.
Plato's unaided mind could handle only simple mathematical
objects, leading to such dichotomies as the idea of a perfect sphere
compared to a mottled, scratched marble ball in the hand. Computer
simulation, like a telescope for the mind's eye, extends mental vision
beyond the nearby realm of simple mathematical objects to distant
worlds, some as complex as physical reality, potentially full of
living beings, warts, minds, and all. Our own world is among this
vista of abstractly conceivable ones, defined by the formal relationships
we call physical law as any simulation is defined by its internal
rules. The difference between physical and mathematical reality is
an illusion of vantage point: the physical world is simply the
particular abstract world that happens to contain us.

The Platonic position on simulation puts a handle on the vexingly
intangible. It holds that every interpretation of a process is a
reality in its own right. Without it an interpretation is meaningful
only in context of another interpretation defining a containing world,
and so on, in an infinite regress. The Platonic position defuses
various worries about intelligent machinery. Some critics argue that
a machine cannot contain a mind since a machine's function is entirely
an outside interpretation, unlike human minds, which supply their own
sense of meaning. The Platonic position on simulation answers that
the abstract relationships that constitute the mind, including its own
self-interpretation, exist independently, and a robot, a simulator, or
a book describing the action, no less than a biological brain, is just
a way of peeking at them. Other critics worry that future robots may
act like intelligent, feeling beings without having an internal sense
of existence---that they will be unconscious, mindless zombies.
Platonism replies that while there are indeed interpretations of any
mechanism (including the human brain) as mindless, there are others
which allow one to see a real, self-appreciating mind. When a robot
(or a person) behaves as if it has beliefs and feelings, our
relationship with it will usually be facilitated if we choose a ``has
a mind'' interpretation. Of course, when working on the internals, a
robotics engineer (or a brain surgeon) may be best served by
temporarily slipping into a ``mindless mechanism'' interpretation.

Platonism puts on the same footing mechanical simulations that
precisely mimic every interaction detail, rough approximations,
cinematic reconstructions, literary descriptions, idle speculation,
dreams, even random gibberish: all can be interpreted as images of
realities; the more detailed presentations simply have a sharper
focus, blurring together fewer alternative worlds. But isn't
there a huge difference between a conventional ``live''
simulation of a world and a simulation transformed to nothing,
requiring a ``recorded'' book or movie to relate the
unfolding events? Isn't it possible to interact with a running
simulation, poking one's finger into the action, in a way
impossible with a static script? In fact, a meaningful interaction
is possible in either case only via an interpretation that connects
the simulated world to the outside. In an interactive simulation,
the viewing mechanism is no longer passive and superfluous, but an
essential bidirectional conduit that passes information to and from
the simulation. Such a conduit can exist for books and movies if
they contain alternative scenarios for possible inputs.
``Programmed learning'' texts popular in previous decades
were of this form, with instructions like ``If you answered A, go
to page 56; if you answered B, go to page 79 . . .'' Some
laser-disc video games give the impression of interactive simulation
by playing video clips contingent on the player's actions.
Mathematically, any interactive mechanism, even a robot or human, can
be viewed as a compact encoding of a script with responses for all
possible input histories. Platonism holds that the soul is in the
abstract relationships represented, not the mechanics of how they are
encoded.

This position seems to have scary moral implications. If simulation
simply opens windows into Platonic realities, and robots and humans,
no less than books, movies, or computer models, are only images of
those essences, then it should be no worse to mistreat a human, an
animal or a feeling robot than to choose a cruel action in a video
game or an interactive book: in all cases you are simply viewing
preexisting realities. But choices do have consequences for the
person making them because of the mysterious contrivance of physical
law and conscious interpretation that produces single threads of
consciousness with unseen futures and unalterable pasts. By our
choices, we each thread our own separate way through the maze of
possible worlds, bypassing equally real alternatives with equally
real versions of ourselves and others, selecting the world we must
then live in.

So is there no difference between being cruel to characters in
interactive books or video games and people one meets in the street?
Books or games act on a reader's future only via the mind, and actions
within them are mostly reversed if the experience is forgotten.
Physical actions, by contrast, have greater significance because their
consequences spread irreversibly. If past physical events could be
easily altered, as in some time-travel stories, if one could go back
to prevent evil or unfortunate deeds, real life would acquire
the moral significance of a video game. A more disturbing implication
is that any sealed-off activity, whose goings on can be forgotten, may
be in the video game category. Creators of hyperrealistic
simulations---or even secure physical enclosures---containing
individuals writhing in pain are not necessarily more wicked than
authors of fiction with distressed characters, or myself, composing
this sentence vaguely alluding to them. The suffering preexists in
the underlying Platonic worlds; authors merely look on. The
significance of running such simulations is limited to their effect on
viewers, possibly warped by the experience, and by the possibility of
``escapees''---tortured minds that could, in principle, leak out to
haunt the world in data networks or physical bodies. Potential
plagues of angry demons surely count as a moral consequence. In this
light, mistreating people, intelligent robots, or individuals in
high-resolution simulations has greater moral significance than doing
the same at low resolution or in works of fiction not because the
suffering individuals are more real---they are not---but because the
probability of undesirable consequences in our own future is greater.

Universal Existence

Perhaps the most unsettling implication of this train of thought is
that anything can be interpreted as possessing any abstract property,
including consciousness and intelligence. Given the right playbook,
the thermal jostling of the atoms in a rock can be seen as the
operation of a complex, self-aware mind. How strange.
Common sense screams that people have minds and rocks don't. But
interpretations are often ambiguous. One day's unintelligible sounds
and squiggles may become another day's meaningful thoughts if one
masters a foreign language in the interim. Is the Mount Rushmore
monument a rock formation or four presidents' faces? Is a
ventriloquist's dummy a lump of wood, a human simulacrum, or a
personality sharing some of the ventriloquist's body and mind? Is a
video game a box of silicon bits, an electronic circuit flipping its
own switches, a computer following a long list of instructions, or a
large three-dimensional world inhabited by the Mario Brothers and
their mushroom adversaries? Sometimes we exploit offbeat
interpretations: an encrypted message is meaningless gibberish except
when viewed through a deliberately obscure decoding. Humans have
always used a modest multiplicity of interpretations, but computers
widen the horizons. The first electronic computer was developed by
Alan Turing to find ``interesting'' interpretations of wartime
messages radioed by Germany to its U-boats. As our thoughts become
more powerful, our repertoire of useful interpretations will grow. We
can see levers and springs in animal limbs, and beauty in the aurora:
our ``mind children'' may be able to spot fully functioning
intelligences in the complex chemical goings on of plants, the
dynamics of interstellar clouds, or the reverberations of cosmic
radiation. No particular interpretation is ruled out, but the space
of all of them is exponentially larger than the size of individual
ones, and we may never encounter more than an infinitesimal fraction.
The rock-minds may be forever lost to us in the bogglingly vast sea of
mindlessly chaotic rock-interpretations. Yet those rock-minds make
complete sense to themselves, and to them it is we who are lost in
meaningless chaos. Our own nature, in fact, is defined by the tiny
fraction of possible interpretations we can make, and the astronomical
number we can't.

Everything and Nothing

There is no content or meaning without selection. The realm of all
possible worlds, infinitely immense in one point of view, is vacuous
in another. Imagine a book giving a detailed history of a world
similar to ours. The book is written as compactly as possible: rote
predictable details are left as homework for the reader. But even
with maximal compression, it would be an astronomically immense tome,
full of novelty and excitement. This interesting book, however, is
found in ``the library of all possible books written in the Roman
alphabet, arranged alphabetically''---the whole library being
adequately defined by this short, boring phrase in quotes. The library
as a whole has so little content that getting a book from it takes as
much effort as writing the book. The library might have stacks
labeled A through Z, plus a few for punctuation, each
forking into similarly labeled substacks, those forking into
subsubstacks, and so on indefinitely. Each branchpoint holds a book
whose content is the sequence of stack letters chosen to reach it.
Any book can be found in the library, but to find it the user must
choose its first letter, then its second, then its third, just as one
types a book by keying each subsequent letter. The book's content
results entirely from the user's selections; the library has no
information of its own to contribute.

Although content-free overall, the library contains individual books
with fabulously interesting stories. Characters in some of those
books, insulated from the vast gibberish that makes the library
worthless from outside, can well appreciate their own existence. They
do so by perceiving and interpreting their own story in a consistent
way, one that recognizes their own meaningfulness---a prescription
that is probably the secret of life and existence, and the reason we
find ourselves in a large, orderly universe with consistent physical
laws, possessing a sense of time and a long evolutionary history.

The set of all possible interpretations of any process as
simulations is exactly analogous to the content of all the books in
the library. In total it contains no information, yet every
interesting being and story can be found within it.

Universal Appreciation

If our world distinguishes itself from the vast unexamined
(and unexaminable) majority of possible worlds through the act of
self-perception and self-appreciation, just who is doing all the
perceiving and appreciating? The human mind may be up to
interpreting its own functioning as conscious, so rescuing itself
from meaningless zombiehood, but surely we few humans and other
biota---trapped on a tiny, soggy dust speck in an obscure corner,
only occasionally and dimly aware of the grossest features of our
immediate surroundings and immediate past---are surely insufficient
to bring meaning to the whole visible universe, full of unimagined
surprises, 10^40
times as massive, 10^70
times as voluminous, and 10^10
times as long-lived
as ourselves. Our present appreciative ability seems more a match
for the simplicity of Saturday-morning cartoons.

The book The Anthropic Cosmological Principle, by
cosmologists John Barrow and Frank Tipler, and Tipler's recent
The Physics of Immortality argue that the crucial parts of the
story lie in our future, when the universe will be shaped more by the
deliberate efforts of intelligence than the simple, blind laws of
physics. In their future cosmology, consistent with the one in this
book, human-spawned intelligence will expand into space, until the
entire accessible universe is inhabited by a cohesive mind that
manipulates events, from the quantum-microscopic to the
universe-macroscopic, and spends some of its energy recalling the
past. Tipler and Barrow predict that the universe is closed: massive
enough to reverse its present expansion in a future ``big
crunch'' that mirrors the big bang. The universe mind will
thrive in the collapse, perhaps by encoding itself into the cosmic
background radiation. As the collapse proceeds, the radiation's
temperature, and so its frequencies and the mind's speed, rise
and there are ever more high-frequency wave modes to store
information. By very careful management, avoiding ``event
horizons'' that would disconnect its parts and using
``gravitational shear'' from asymmetries in the collapse to
provide free energy, Tipler and Barrow calculate that the cosmic mind
can contrive to do more computation and accumulate more memories in
each remaining half of the time to the final singularity than it did
in the one before, thus experiencing a neverending infinity of time
and thought. As it contemplates, effects from the universe's
past converge on it. There is information, time, and thought enough
to recreate, savor, appreciate, and perfect each detail of each
moment. Tipler and Barrow suggest that it is this final,
subjectively eternal act of infinite self-interpretation that
effectively creates our universe, distinguishing it from the others
lost in the library of all possibilities. We truly exist because our
actions lead ultimately to this ``Omega Point'' (a term
borrowed from the Jesuit paleontologist and radical philosopher
Tielhard de Chardin).

Uncommon Sense

Although our eyes and arms effortlessly predict the liftability of a
rock, the action of a lever, or the flight of an arrow, mechanics was
deeply mysterious to those overly thoughtful ancients who pondered
why stones fell, smoke rose, or the moon sailed by unperturbably.
Newtonian mechanics revolutionized science by precisely formalizing
the intelligence of eye and muscle, giving the Victorian era a
viscerally satisfying mental grip on the physical world. In the
twentieth century, this common-sense approach was gradually extended
to biology and psychology. Meanwhile, physics moved beyond common
sense. It had to be reworked because, it turned out, light did not
fit the Newtonian framework.

In a one-two blow, intuitive notions of space, time, and reality
were shattered, first by relativity, where space and time vary with
perspective, then more seriously by quantum mechanics, where
unobserved events dissolve into waves of alternatives. Although
correctly describing everyday mechanics as well as such important
features of the world as the stability of atoms and the finiteness of
heat radiation, the new theories were so offensive to common sense,
in concept and consequences, that they inspire persistent
misunderstandings and bitter attacks to this day. The insult will
get worse. General relativity, superbly accurate at large scales and
masses, has not yet been reconciled with quantum mechanics, itself
superbly accurate at tiny scales and huge energy concentrations.
Incomplete attempts to unite them in a single theory hint at
possibilities that exceed even their individual strangeness.

The strangeness begins just beyond the edges of the everyday world.
When an object travels from one place to another, common sense
insists that it does so on a definite, unique trajectory. Not so,
says quantum mechanics. A particle in unobserved transit goes every
possible way simultaneously until it is observed again. The
indefiniteness of the trajectory manifests itself in the kind of
interference pattern created by waves that spread and recombine,
adding where they meet in step and canceling where out of step. A
photon, a neutron, or even a whole atom sent to a row of detectors
via a screen with two slits, will always miss certain detectors,
where the wave of its possible positions, having passed through
both slits, cancels.

Experimental results forced the quantum view of the world on
reluctant physicists piecemeal during the first quarter of the
twentieth century and it still has ragged edges. The theory is neat
in describing the unobserved, where, for instance, a particle spreads
like a wave. It fails to define or pinpoint the act of observation,
when the ``wave function'' collapses and the particle appears
in exactly one of its possible places, with a probability given by
the intensity of its wave there. It may be when the detector
responds, or when the instrumentation connected to the detector
registers, or when the experimenter notes the instrument readings, or
even when the world reads about the result in physics journals!

In principle, if not practice, the point of collapse can be
pinpointed: before collapse, possibilities interfere like waves,
creating interference patterns; after collapse, possibilities simply
add in a common-sense way. Very small objects, like neutrons
traveling through slits, make visible interference patterns.
Unfortunately, large, messy objects like particle detectors or
observing physicists would produce interference patterns much, much
finer than atoms, indistinguishable from common-sense probability
distributions because they are so easily blurred by thermal
jiggling.

Because, for humans, common sense is easier than quantum theory,
workaday physicists take collapse to happen as soon as
possible---for instance, when a particle first encounters its
detector. But this ``early collapse'' view can have peculiar
implications. It implies that the wave function can be repeatedly
collapsed and uncollapsed in subtle experiments that allow
measurements to be undone through deliberate cancellation at the
experimenter's whim.

This wave function yo-yo is less problematical if one assumes that
the collapse happens further downstream where it is more difficult to
undo the measurement. Just where the hope of reversal ends is a
moving target, as quantum experiments become ever more controlled and
subtle. Einstein was troubled by the implications of quantum
mechanics, and he devised thought experiments with outcomes so
counterintuitive he felt they discredited the theory. Those
counterintuitive outcomes are now observed in laboratories and
utilized in experimental quantum computers and cryptographic
signaling systems. Soon, more advanced quantum computers will allow
the results of entire long computations to be undone.

Common sense screams that measurements are real when they register in
the experimenter's consciousness. This thinking has led some
philosophically inclined physicists to suggest that consciousness
itself is the mysterious wave-collapsing process that quantum theory
fails to identify. But even consciousness is insufficient to cause
collapse in the thought experiment known as ``Wigner's Friend.'' Like
the more famous ``Schrödinger's Cat,'' Wigner's friend is sealed
in a perfectly isolating enclosure with a physics experiment that has
two possible outcomes. The friend observes the experiment and notes
the outcome mentally. Outside the leakproof enclosure, Wigner can
only describe his friend's mental state as the superposition of the
alternatives. In principle these alternatives should interfere, so
that when the enclosure is opened one or another outcome may be
favored, depending on the precise time of opening. Wigner might then
conclude that his own consciousness triggered the collapse when the
enclosure was opened, but his friend's earlier observation had left it
uncollapsed.

Assuming that effects behave quantum mechanically until some point
when their wave functions become so entangled with the world that they
are beyond hope of reversal, at which point they behave
commonsensically, eliminates philosophical problems for most
laboratory physicists. It creates problems for cosmologists, whose
scope is the entire universe, for it implies the world is peppered
with collapsed wave functions surrounding observing devices. These
collapses have no theory and cannot be experimentally quantified and
thus make it impossible to set up equations for the universe
overall. Instead, cosmologists assume the entire universe behaves as
a giant wave function that evolves according to quantum theory and
never collapses. But how can a ``universal wave function,''
in which every particle forever spreads like a wave, be reconciled
with individual experiences of finding particles in particular
positions?

Many Worlds

In a 1957 Ph.D. thesis, Hugh Everett gave a new answer to that
question. Given a universally evolving wave function, where the
configuration of a measuring apparatus, no less than of a particle,
spreads wavelike through its space of possibilities, he showed that if
two instruments recorded the same event, the overall wave function had
maximum magnitude for situations where the records concurred and
canceled where they disagreed. Thus, a peak in the combined wave
represents a possibility where, for instance, an instrument, an
experimenter's memory, and the marks in a notebook agree on where a
particle alighted---eminent common sense. But the whole wave function
contains many such peaks, each representing a consensus on a different
outcome. Everett had shown that quantum mechanics, stripped of
problematical collapsing wave functions, still predicts common-sense
worlds---only many, many of them, all slightly different. The
``no-collapse'' view became known as the ``many-worlds''
interpretation of quantum mechanics. Its implication that each
observation branched the world into something like $10^{100}$ separate
experiences seemed so extravagantly insulting to common sense that it
was passionately rejected by many. Although cosmologists worked with
the universal wave function, its connection to the everyday world was
ignored for another twenty years.

Recent subtle experiments confirming the most mind-bending
predictions of quantum mechanics, including the development of
quantum computers, have lifted many-worlds' stock relative to
traditional interpretations that require influences to leap wildly
across time and space to explain the observed correlations. The
theoretical trail pioneered by Everett is becoming traveled and
extended. Since the late 1980s James Hartle and Murray Gell-Mann
have investigated its underlying notions of measurement and
probability.

Everett had demonstrated that the conventional rules for collapsing
the wave function to measurement-outcome probabilities from
``outside'' a system were consistent with what would be
reported by (each version of) the uncollapsed observer
``inside,'' thus removing the requirement for an outside or a
collapse and raising our consciousness to existence of many worlds.
He made no attempt to show how those peculiar measurement rules arose
in the first place. Gell-Mann and Hartle are asking this difficult
question. They are far from a final resolution, but their work so
far shows just how special---or illusory---the common-sense
world really is.

Hartle and Gell-Mann note that if we were to try to observe and
remember events at the finest possible detail---around $10^{-30}$
centimeters, far
smaller than anything reachable today---the interference of all
possible worlds would present a seething chaos with no permanent
structures, no quiet place to store memories, effectively no
consistent time. At a coarser viewing scale---$10^{-15}$
centimeters, the
submicroscopic world touched by today's high-energy
physics---much of the chaos goes unobserved, and multiple worlds
merge together, canceling the wildest possibilities, leaving those
where particles can exhibit a consistent existence and motion, if
still jaggedly unpredictable, through a vacuum that boils with
ephemeral ``virtual'' energy. Everyday objects have the
smooth, predictable trajectories of common sense only because our dim
senses are coarser still, registering nothing finer than $10^{-5}$
centimeters. At
scales larger than the everyday (or the Hartle--Gell-Mann
analysis), the events we consider interesting are blurred to
invisibility, and the universe is increasingly boring and
predictable. At the largest possible scale, the universe's
matter is canceled by the negative energy in its gravitational fields
(which strengthen while releasing energy, as matter falls together),
and in sum there is nothing at all.

No complete theory yet explains our existence and experiences, but
there are hints. Tiny universes simulated in today's computers
are often characterized by adjustable rules governing the interaction
of neighboring regions. If the interactions are made very weak, the
simulations quickly freeze to bland uniformity; if they are very
strong, the simulated space may seethe intensely in a chaotic boil.
Between the extremes is a narrow ``edge of chaos'' with
enough action to form interesting structures, and enough peace to let
them persist and interact. Often such borderline universes can
contain structures that use stored information to construct other
things, including perfect or imperfect copies of themselves, thus
supporting Darwinian evolution of complexity. If physics itself
offers a spectrum of interaction intensities, it is no surprise that
we find ourselves operating at the liquid boundary of chaos, for we
could not function, nor have evolved, in motionless ice nor formless
fire.

The odd thing about the Hartle--Gell-Mann spectrum is that it is
not some external knob that controls the interaction intensity, but
varying interpretations of a single underlying reality made by
observers who are part of the interpretation. It is, in fact, the
same kind of self-interpretation loop we encountered when considering
observers inside simulations. We are who we are, in the world we
experience, because we see ourselves that way. There are almost
certainly other observers in exactly the same regions of the wave
function who see things entirely differently, to whom we are simply
meaningless noise.

The similarity between Everett's ``many worlds'' and
the philosophical ``possible worlds'' may become stronger
yet. In ``many worlds'' quantum mechanics, physical
constants, among other things, have fixed values. Gravity, in
objects like black holes, loosens the rules, and a full quantum
theory of gravity may predict possible worlds far exceeding
Everett's range---and who knows what potent subtleties lie
even further on? It may turn out, as we claw our way out through
onion layers of interpretation, that physics places fewer and fewer
constraints on the nature of things. The regularities we observe may
be merely a self-reflection: we must perceive the world as compatible
with our own existence---with a strong arrow of time, dependable
probabilities, where complexity can evolve and persist, where
experiences can accumulate in reliable memories, and the results of
actions are predictable. Our mind children, able to manipulate their
own substance and structure at the finest levels, will probably
greatly transcend our narrow notions of what is.

Questioning Reality

Like organisms evolved in gentle tide pools, who migrate to
freezing oceans or steaming jungles by developing metabolisms,
mechanisms, and behaviors workable in those harsher and vaster
environments, our descendants may develop means to venture far from
the comfortable realms we consider reality into arbitrarily strange
volumes of the all-possible library. Their techniques will be as
meaningless to us as bicycles are to fish, but perhaps we can stretch
our common-sense-hobbled imaginations enough to peer a short distance
into this odd territory.
Physical quantities like the speed of light, the attraction of
electric charges, and the strength of gravity are, for us, the
unchanging foundation on which everything is built. But if our
existence is a product of self-interpretation in the space of all
possible worlds, this stability may simply reflect the delicacy of
our own construction---our biochemistry malfunctions in worlds
where the physical constants vary, and we would cease to be there.
Thus, we always find ourselves in a world where the constants are
just what is needed to keep us functioning. For the same reason, we
find the rules have held steady over a long period, so evolution
could accumulate our many intricate, interlocking internal
mechanisms.

Our engineered descendants will be more flexible. Perhaps
mind-hosting bodies can be constructed that are adjustable for small
changes in, say, the speed of light. An individual who installed
itself in such a body, and then adjusted it for a slightly higher
lightspeed, should then find itself in a physical universe
appropriately altered, since it could then exist in no other. It
would be a one-way trip. Acquaintances in old-style bodies would be
seen to die---among fireworks everywhere, as formerly stable atoms and
compounds disintegrated. Turning the tuning knob back would not
restore the lost continuity of life and substance. Back in the old
universe everything would be normal, only the acquaintances would
witness an odd ``suicide by tuning knob.'' Such irreversible partings
of the way occur elsewhere in physics. The many-worlds interpretation
calls for them, subtly, at every recorded observation. General
relativity offers dramatic ``event horizons'': an observer falling
into a black hole sees a previously inaccessible universe ahead at the
instant she permanently loses the ability to signal friends left
outside.

Visiting offbeat worlds, where the dependable predictability of the
common sense no longer holds, is probably much too tricky for crude
techniques like the last paragraph's knob turning. It must be far
more likely that mechanical fluctuations or other effects persistently
frustrate attempts to retune a body than for physical constants to
actually change. Yet once our descendants achieve fine-grain mastery
of extensive regions of the universe, they may be able to orchestrate
the delicate adjustments needed to navigate deliberately among the
possibilities, perhaps into difficult but potent regions shaped by
interrelationships richer than those of matter, space, and time. Time
travel, a technology faintly visible on our horizon, may mark merely
the first and most pedestrian route in this limitless space.

Until Death Do Us Part

We can't yet leave the physical world in chosen directions,
but we are scheduled to leave it soon enough in an uncontrolled way
when we die. But why do we seem so firmly locked to the simple
physical laws of the material world before death? This is a most
fundamental question if one accepts that all possible worlds are
equally real. Artificial intelligence programs, which recreate the
psychological state of nervous systems without simulating the
detailed physical substance that underlies them, and virtual
realities, which allow unphysical magical effects like teleportation,
suggest that our own consciousnesses can exist in many possible
worlds that do not follow our physical laws. This question of why
our universe seems so firmly yoked to physical law has hardly been
asked in a scientific way, let alone answered. But the answer may be
related to Einstein's observation that mathematics seems to be
unreasonably effective in describing the physical world. This
unreasonableness shows itself in the plausible, already partially
fulfilled, quest of physics for a ``Theory of Everything,''
perhaps a simple differential equation whose solution implies our
whole physical universe and everything in it!

In our daily meanders, we are more likely to stumble across a
particular small number (say ``5'') than a particular large one (say
``53783425456''). The larger number requires far more digits to
simultaneously fall into place just so, and thus is far less likely.
Similarly, although we exist in many of all possible universes, we are
most likely to find ourselves in the simplest of those, the few that
require the least number of things to be just so. The universe's
great size and age, its physical laws, and our own long evolution may
be just the working of the simplest possible rules that produce our
minds.

Our consciousness now finds itself dependent on the operation of
trillions of cells tuned exquisitely to the physical laws into which
we evolved. It continues from moment to moment most simply if those
laws continue to operate as they have in the past. Thus, with
overwhelming probability, we find the laws are stable. In the space
of all possible universes, we are bound to the same old one. As long
as we remain alive.

When we die, the rules surely change. As our brains and bodies
cease to function in the normal way, it takes greater and greater
contrivances and coincidences to explain continuing consciousness by
their operation. We lose our ties to physical reality, but, in the
space of all possible worlds, that cannot be the end. Our
consciousness continues to exist in some of those, and we will always
find ourselves in worlds where we exist and never in ones where we
don't. The nature of the next simplest world that can host us,
after we abandon physical law, I cannot guess. Does physical reality
simply loosen just enough to allow our consciousness to continue? Do
we find ourselves in a new body, or no body? It probably depends
more on the details of our own consciousness than did the original
physical life. Perhaps we are most likely to find ourselves
reconstituted in the minds of superintelligent successors, or perhaps
in dreamlike worlds (or AI programs) where psychological rather than
physical rules dominate. Our mind children will probably be able to
navigate the alternatives with increasing facility. For us, now,
barely conscious, it remains a leap in the dark. Shakespeare's
words, in Hamlet's famous soliloquy, still apply:

To die, to sleep;
To sleep: perchance to dream: ay, there's the rub;
For in that sleep of death what dreams may come
When we have shuffled off this mortal coil,
Must give us pause: there's the respect
That makes calamity of so long life;
For who would bear the whips and scorns of time,
The oppressor's wrong, the proud man's contumely,
The pangs of despised love, the law's delay,
The insolence of office and the spurns
That patient merit of the unworthy takes,
When he himself might his quietus make
With a bare bodkin? who would fardels bear,
To grunt and sweat under a weary life,
But that the dread of something after death,
The undiscover'd country from whose bourn
No traveller returns, puzzles the will
And makes us rather bear those ills we have
Than fly to others that we know not of?
Thus conscience does make cowards of us all;
And thus the native hue of resolution
Is sicklied o'er with the pale cast of thought,
And enterprises of great pith and moment
With this regard their currents turn awry,
And lose the name of action.